require sustained exposure and practice in school-aged children, such as specialized vocabulary and uses of the subjunctive in Spanish. Attrition An incomplete grammar may be the result of attrition and fossilization of concepts in the L1 due to insufficient input once the child has switched to the dominant language. Attrition is the loss of, or failure to make full use of, “grammatical knowledge previously acquired". Fossilization occurs when a speaker lets go of non-core grammatical concepts but retains the basic core structure of the language. Attrition and fossilization are considered to be part of the Incomplete Acquisition Theory because they render a language incomplete. However, Pires and Rothman (2009) argue that attrition should be distinguished from the Incomplete Acquisition Theory in future studies because, in instances of attrition, the speakers do, indeed, completely acquire the target grammar as children. Alternative acquisition theories Delayed Acquisition Delayed acquisition may contribute to the difference in language competence between heritage speakers and monolingual speakers of the same language. Late or delayed acquisition of a language can have "consequences for linguistic, neurological, and cognitive mechanisms" that work to make language acquisition "fast, effortless, and... successful". Flores and Barbosa (2014) studied the clitic placement of heritage speakers of Portuguese living in Germany and concluded that the heritage speakers went through the same stages of acquisition as their monolingual counterparts. However, the process of acquisition was delayed and took longer for the heritage speakers because they had reduced input of the heritage language. Thus, this theory must be considered in conjunction with theories on input variation. Input Theories of acquisition involving variations of input postulate that heritage language learners' production of the heritage language diverges from that of their monolingual peers who speak the same language because the two groups are exposed to different dialects and quantities of input. Dialect Variation Theories surrounding dialect variation suggest that errors or deviations from the standard dialect made by a heritage language learner may reflect the acquisition of a non-standard variety or informal register of the heritage language, which include variations on certain properties or the lack of certain properties found in the standard dialect. Heritage language learners are often only exposed to one dialect or colloquial variety of the heritage language, unlike their monolingual peers who interact with a standard monolingual dialect found in formal instruction. Furthermore, certain grammatical properties are only present in the standard dialect or are